Field of the Invention
This invention relates to a method for producing liquid organic fuels from hydrocarbons. In one aspect, this invention relates to an electrochemical method for producing liquid organic fuels from hydrocarbons. In one aspect, this invention relates to electrochemical devices for producing liquid organic fuels from hydrocarbons. In one aspect, this invention relates to an electrochemical method for producing hydrogen.
Description of Related Prior art
Methane is an abundant fuel especially with the development and production of shale gas. However, it is underutilized as a precursor for chemicals and liquid fuels due to the difficulty of transporting the gas, particularly with scattered shale gas supplies. Methanol is one of the 25 top chemicals produced worldwide; it is the main feedstock for the chemical industry; and it is a source of dimethyl ether (DME), which could be used as a vehicular fuel.
Methanol is conventionally produced from a Fischer-Tropsch reaction via the high temperature steam reforming of methane followed by high pressure reaction of the reformate hydrogen and CO. The efficiency is only about 50-65% depending on the waste heat recovery. Thus, highly efficient and cost effective conversion of methane to methanol is very much desired. While there have been considerable attempts to oxidize methane to methanol over solid catalysts in single-pass processes, none have been recognized to be practical. The reasons are: 1) methane is quite inert and generally requires temperatures greater than about 400° C. to react; and 2) methanol is produced as an intermediate product, thus limiting yield and selectivity. Some studies have reported the direct conversion of methane to methanol at slightly lower temperatures, still typically greater than about 300° C., using oxidants stronger than oxygen, such as N2O, hydrogen peroxide, and ozone. However, such oxidants are not practical for high volume production due to cost.
FIGS. 1a-1c show a comparison of known technologies for direct conversion of methane to methanol at low temperatures. As used herein, the term “low temperature” refers to temperatures less than or equal to about 160° C. These technologies include a fuel cell type reaction, an internal short fuel cell type design and a metal oxide cation mediated reaction.
The electrochemical method to convert methane to methanol uses the concept that a strong oxidative O* radical intermediate is generated at the cathode electrode in a fuel cell type reactor (FIG. 1a) or in an internal short fuel cell type reactor (FIG. 1b). In this reactor, hydrogen gas is supplied to the anode side to produce protons and oxygen at the cathode side reacts with the protons from the anode to generate hydrogen peroxide intermediates, which contain strong oxidative O* species. Methane then reacts with the O* species to produce methanol. U.S. Pat. No. 5,051,156 teaches that methane activation may be performed at the anode and oxygen feeding at the cathode as a fuel cell reaction, which is quite similar to a high temperature solid oxide fuel cell with partial oxidation.
To avoid the use of hydrogen, it is known to use an intermediate temperature system. In this system, water, oxygen, and methane are supplied to a reactor with an applied galvanic current. Water is decomposed to oxygen and protons at the anode and methane reacts with oxygen and protons at the cathode. The methane is oxidized to methanol at temperatures greater than about 300° C.
Transition-metal oxide cations have been used to oxidize C—H and C—C bonds to produce more valuable products. The direct oxidation of methane to methanol is the simplest oxidation process (FIG. 1c). It is known that metal oxide cations can convert methane to methanol directly. Suitable metal cations include Mn, Fe, Co, Ni, Ru, Rh, Ir, Pt, and possibly others. However, the direct conversion has several issues including the requirement for oxygen separation from air, low product yield, and low selectivity for the desired product.
Metal oxide cations, MO+, may be produced by oxygen reacting with M+ in accordance with the following reactions:M++½O2→MO+MO++CH4→CH3OH+M+However, this chemical reaction is limited by the formation of M+ and MO+ and continuous regeneration of the MO+ catalyst is required to continuously produce the liquid methanol with high selectivity. The most reactive ions are MnO+, FeO+, NiO+, OsO+, and PtO+. The metal oxide cations react with methane to produce methanol. However, these reactive metal oxide cations require pure oxygen for regeneration. Ideally, the direct methods should have an economic advantage over indirect methods, but to date, no direct processes have progressed to a commercially acceptable stage, largely due to low product yields.
Metal oxide cations have been investigated as catalytic intermediates to oxidize methane to produce liquid methanol and the methane to methanol conversion using metal oxide cations which selectively produce methanol by oxidizing methane at low temperatures has been reported. This reaction has been investigated using computational modeling and spectroscopic monitoring to find the reaction mechanisms. However, no practical industrial reactors have been built due to the catalyst lifetime, reactivity, mass transfer limitations, and selectivity.
Table 1 contains a comparison of the known low temperature technologies for direct conversion of methane to methanol.
TABLE 1Comparison of low temperature technologies for methaneto methanol conversionAnodeCathodeFuel cell typeHydrogen feed toOxygen feed to reactproduce protonswith protons from anodeand produce peroxideintermediate, whichreacts with methane toproduce methanolMicro-cellThis is an internal shortOxygen produced on sitefuel cell type particlereacts with methane toreactionproduce methanolMetal oxide cationOxygen generated metal oxide cations react withmethane to form methanol. The oxygen needs to beseparated from air.